A flexible printed circuit board (FPCB) is a printed circuit board having a base material composed of polyester resin or polyimide resin. The flexible printed circuit board can be variously bent and folded according to the needs of any particular application. The flexible printed circuit board has a small volume, and features good heat dissipation performance and ease of assembly.
A conventional method for making an FPCB includes the following steps: selecting a base material; imaging in an inner layer of the base material; surface treatment; laminating; drilling; pretreating; hole metallization; imaging; pattern plating; releasing a formed film; etching; and machining outer shape of FPCB. A rate of production of the FPCB directly depends on a rate of drilling. Further, the quality of drilled via holes directly determines the quality and performance of the FPCB.
Conventionally, there are two kinds of methods for drilling via holes in an FPCB: a mechanical drilling method, and a laser drilling method. The mechanical drilling method utilizes a small drill bit to machine via holes in the FPCB. The mechanical drilling method is generally regarded as time-consuming. In addition, the quality of the via holes made in the FPCB by the mechanical drilling method may be unsatisfactory. In addition, some debris may remain in the via holes. This can result in short circuits or open circuits in the electric circuits of the FPCB. The laser drilling method can overcome many or even all of the above-mentioned problems associated with the mechanical drilling method. Via holes with small diameters can be efficiently obtained via the laser drilling method.
A conventional laser drilling system is shown in FIG. 1. A laser beam 12 from a CO2 laser source 11 sequentially passes through a collimating lens 131 and a screen 132 of an optic image transfer system 13, and is incident on a galvano-mirror 14. The reflected laser beam 12 from the galvano-mirror 14 passes through an fθ lens 15, and then is incident on the FPCB 161 placed on a working table 16. Via holes are thereby defined in the FPCB 161.
Thus, the via holes with small diameters are obtained by the laser drilling method using a CO2 laser source. However, the laser beam generated by the CO2 laser source is in a form of a long pulse. The percentage of unstable pulses is generally more than 2%, which makes the drilling process unstable and may diminish the uniformity of the via holes. In addition, the long wavelength and high power of the laser beam may result in relatively low precision of the via holes. Furthermore, some debris may remain in the via holes. If this occurs, special treatment for removing the debris may be required.
What is needed, therefore, is to provide a new laser drilling system and method for drilling via holes in an FPCB, which system and method can overcome the above-described problems.